One type of prior art is a rotation detection arrangement, shown and described in published Japanese Patent Application No. 59-6445. An illustrative embodiment of such a speed sensing system is explained below with reference to FIGS. 2a and 2b of the accompanying drawings.
As shown, the outer circumference of a wheel axle AX is provided with a number of permanent magnets M, which are arranged at essentially the same spaced intervals. The magnets are arranged such that they have the same polar orientation. A suitable covering material or OT cover tape is used. The circumferential distance A between adjacent permanent magnets M is greater than the width B of the magnets M, so that A&gt;B. The sum of the distance A and the width B around the circumference determines the installation interval or pitch P wherein P=(A+B). Magnetic head A or holder H is suitably fixed to the main body of the motor MB by a bracket B. On this holder, there is mounted magnetic detecting or sensing element GS which is spaced in relationship to the axle AX.
Since the holder H is mounted on the sprung portion of the vehicle, it fluctuates vertically as a result of the vibrational movement of the vehicle body while in motion. Thus, the distance between the magnets M and the magnetic detection element GS may vary between 20-30 mm. If the magnets M are relatively small, then the strength of the lines of magnetic flux is relatively weak. Thus, the vibrational fluctuations may result in the inability to accurately detect or sense the actual speed or rotational velocity of the axle AX. Conversely, when the magnets M are relatively large, the number of magnets that can be installed on the outside circumference of the axle AX is limited. Therefore, the width B of the permanent magnets M is generally limited to 15-30 mm. Furthermore, the diameter of axle D in railway vehicles is usually between 150-170 mm. The following table 1 illustrates the axle diameter D, the distance A between adjacent magnets, the width B of the magnets, the mounting interval or pitch P, and the maximum number of magnets which may be installed on the circumference of wheel axle to effectively and efficiently detect the rotational speed of a moving vehicle.
TABLE 1 ______________________________________ Distance Width Axle Between of Magnet Maximum Diameter Magnets Magnets Pitch Number of D (mm) A (mm) B (mm) P (mm) Magnets ______________________________________ 150 16.4 15 31.4 15 37.3 30 67.3 7 160 16.4 15 31.4 16 32.9 30 62.9 8 170 16.4 15 31.4 17 36.8 30 66.8 8 ______________________________________
As shown in Table 1, a conventional-type of rotational detection arrangement is only capable of efficaciously accommodating between 7 and 17 magnets, depending on the axle diameter D and the width B of the magnets M.
Therefore, in such conventional-type of magnetic speed detectors, as described above, there will be 7 to 17 pulses generated for each revolution of the axle AX. These pulses are then fed to a suitable pulse measuring instrument PM. The pulses are amplified and shaped to a desired wave form, such as, square-wave pulses. The square-wave pulses are fed to a digital-to-analog converter for conversion to a speed signal which is displayed on the console or like in the cab or compartment of the operator of the railway vehicle or train.
In such a conventional speed detection system, the number of pulses obtainable during one rotation of axle AX is limited to 7 to 17. One of the problems with such a system is that the resolution and accuracy of the speed determination on such limited data is relatively poor.
As a result of various studies of this problem, it was discovered that, in order to increase the resolution and accuracy of the speed measurements to a sufficiently acceptable level, it was necessary to generate sixty (60) or more pulses per revolution of the axle AX.